This invention relates to an improved design of a variable displacement hydraulic unit such as a pump or hydrostatic transmission (“HST”), and in particular to an improved return to neutral feature. Hydrostatic transmissions and other hydraulic units using an axial piston design are well known in the art. While this invention will be generally described in connection with an HST, it is understood that this invention could be applied to a variety of hydrostatic units, such as stand-alone pumps using external hoses. The invention described herein can also be adapted for use in an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles, and a wide variety of uses, including vehicles and industrial applications.
In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself.
The pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The axial pistons of the pump engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing rotation thereof. The axial pistons of the motor are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an IHT, or external thereto. The swash plate is generally controlled by a control arm which is connected via linkage to either a hand control or foot pedal mechanism which the vehicle operator uses to control direction and speed.
The pump system is fully reversible in a standard HST. As the swash plate is moved, the rotational direction of the motor can be changed. The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example.
The hydrostatic pump described herein has a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid. Where the pump pistons move vertically, the swash plate is in neutral when it is generally horizontal with respect to the pump pistons. The swash plate need not be horizontal in the neutral position, depending on the orientation of the pump, but it will be generally perpendicular to the pump pistons in the neutral position.
For safety reasons, and for the convenience of the user, it is preferred to have a return to neutral, or zero displacement, feature, which forces the swash plate to its neutral position when no force is being applied to the control arm. Such devices are important for vehicle safety, to eliminate unintended movement of the vehicle, and to return the unit to neutral in the event of an accident where the vehicle operator is unable to physically disengage the transmission. Such return to neutral devices generally involve a spring mechanism engaged to the control arm to force the control arm to a neutral position, which then returns the swash plate to a neutral position. These may be located external to the housing or internally.
One example of a device used to maintain a hydrostatic unit in the zero displacement mode is shown in U.S. Pat. No. 5,207,144. While that design incorporates a spring mechanism to force a return to neutral, the reciprocal follower used to contact the swash plate does not separately pivot itself, leading to binding problems.